Among prior art devices for deflating tires to a desired level are the tire pressure gauges on the market such as the model S60X from G. H. Meiser & Co. This gauge has a button on the side of the gauge stem to bleed air from the tire to facilitate deflation; the integral gauge is used to stop deflation at the desired point.
U.S. Pat. No. 1,686,659 of Henemer provides a deflating valve cap which screws into the tire valve to bleed air. The tire deflation mechanism of U.S. Pat. No. 4,569,363 of Kleeman is a convertible tire cap that can be unscrewed from a tire valve and screwed back in an inverted configuration where an internal projection interacts with the pin on the tire valve to facilitate bleeding of air for deflation. In both of these devices, a separate gauge must be used to monitor the desired terminal pressure.
U.S. Pat. No. 4,658,869 of Soon-Fu discloses a combination tire inflator, gauge and deflator. It is suitable for attachment to a compressor such as is often available at gas stations. A handle is used to discharge air from a tire. The degree of deflation is indicated on the integral tire pressure gauge.
U.S. Pat. No. 5,115,832 of Higdon et al. shows a tire pressure gauge and cap apparatus where a small tire cap device has an optical window which indicates the presence or absence of a minimum pressure inside the tire. It is not a deflation device.
All of the prior art deflation devices require the use of a visual indicating gauge to manually terminate the deflation process at the desired pressure.